Devices of this general type are used for systems for stabilizing fluctuations in sliver weight and for detecting the quality at cards, carding machines and draw frames. Such systems serve to keep the fluctuations in yarn number or count in the yarn being produced so small that the fluctuations do not spoil the properties in the finished product. The main differences in the known regulating systems lie in the measuring elements employed in them. Essentially three types of these measuring elements are known: the so-called actively pneumatic measuring element; the roller measuring system; and the fiber pressing system. With regard to the first two measuring elements, reference is made to the USTER News Bulletin No. 30, Jun. 1982. With regard to the last-mentioned measuring element, reference is made to U.S. Pat. No. 4,864,853.
In U.S. Pat. No. 4,864,B53, the sliver is scanned by a measuring element formed by a leaf spring. The sliver contacts the leaf spring in a measuring channel which is provided in a measuring part interchangeably arranged on the compaction element. This has the advantage that the entire compaction element does not need to be exchanged in order to adapt the device for measuring work in connection with slivers of different counts. On the contrary, only the measuring part needs to be exchanged. This device has proved excellent in practice, but it has been found that there are certain limits to the measuring accuracy. It may be supposed that this is directly connected with the compaction of the sliver, the so-called filling factor, which might well be limited by the spatial separation of compaction element on the one hand and measuring element on the other hand.
In the roller measuring system, the sliver is compacted by a pair of measuring rollers between which the sliver is pressed together. Here, compaction element and measuring element are not spatially separate; on the contrary, both functions are exercised by the measuring rollers. The two rollers are designed to overlap one another to prevent the sliver from coming laterally out of the clamping gap, and in fact they are designed either as stepped rollers or as so-called grooved and scanning rollers. The grooved- and scanning-roller measuring element is also known by the designation tongue and groove. Although a relatively high compaction of the sliver is obtained with the roller measuring system, this measuring system is very sluggish for this purpose on account of its relatively high mass moment of inertia, so that it is unable to outweigh the advantages of the fiber pressing system described in U.S. Pat. No. 4,864,853.